cmd/go/internal/modload: emit correct module in duplication error

Updates #26904.

Change-Id: If7f381c3f3a41bd62c5f8bcf4f92720badcaf5c6
Reviewed-on: https://go-review.googlesource.com/128878
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/internal/modload/load.go

@@ -140,7 +140,7 @@ func ImportPaths(args []string) []string {
 		if prev, ok := firstPath[src]; !ok {
 			firstPath[src] = mod.Path
 		} else if prev != mod.Path {
-			base.Errorf("go: %s@%s used for two different module paths (%s and %s)", mod.Path, mod.Version, prev, mod.Path)
+			base.Errorf("go: %s@%s used for two different module paths (%s and %s)", src.Path, src.Version, prev, mod.Path)
 		}
 	}
 	base.ExitIfErrors()

src/cmd/go/testdata/script/mod_replace.txt

@@ -20,7 +20,7 @@ stdout 'Clear is better than clever.'
 # However, the same module can't be used as two different paths.
 go mod edit -dropreplace=rsc.io/quote/v3 -replace=not-rsc.io/quote/v3@v3.0.0=rsc.io/quote/v3@v3.0.0 -require=not-rsc.io/quote/v3@v3.0.0
 ! go build -o a4.exe .
-
+stderr 'rsc.io/quote/v3@v3.0.0 used for two different module paths \(not-rsc.io/quote/v3 and rsc.io/quote/v3\)'
 
 -- go.mod --
 module quoter